Uniformly charged rod, Physics

a) Three charges, every equal to 1 micro coulomb, are placed at the three corners of a square of side 1 m. Find the electric field at the fourth corner.

b) Determine the electric filed at a point P on the perpendicular bisector of a uniformly charged rod. The length of the rod is L, the charge on it is 'Q' and the distance of P from the centre of the rod is 'a'.